1. Field of the Invention
The present disclosure relates generally to photocatalysts made of nanocrystallites of titanium dioxide (TiO2). More particularly, the TiO2 nanocrystallites are less than 14 nanometers (nm) in diameter, and treated to minimize the internal defects within the TiO2 nanocrystallites. The present disclosure provides a process for preparing and a method of using such nanocrystalline TiO2 photocatalysts to purify air, water, or other fluids, by photocatalyzing contaminants.
2. Description of the Related Art
Photocatalytic Oxidation (PCO) is a technology used for elimination or reduction of the level of contaminants in fluids such as air or water (or other fluids) using the chemical action of light. When ultraviolet (UV) light is used to energize the photocatalyst, the technology is more specifically termed Ultraviolet Photocatalytic Oxidation (UV-PCO).
Indoor air can include trace amounts of contaminants, including carbon monoxide, ozone, and volatile organic compounds (VOC), such as formaldehyde, toluene, propanal, butene, propionaldehyde and acetaldehyde. Air purifiers using UV-PCO technology can be used to chemically convert these contaminants into less-harmful products, such as carbon dioxide and water, and/or less-polluting products that are more easily removed from the air than their parent compounds.
Other methods have been used to remove contaminants from air. Absorbent air filters, which use absorbent materials such as but not limited to activated carbons, clays, or mesoporous zeolites, remove contaminants from air by trapping the contaminants in the pores of the filter and permitting cleaner air to pass through the filter. An obvious drawback of absorbent filters is that such filters merely block or trap contaminants and the filters are susceptible to clogging, and absorbent air filters often cannot effectively remove certain types of airborne contaminants, such as ozone or carbon monoxide.
Photocatalysts having semiconductor activity have been used in air purification systems for elimination of organic contaminants in air, including titanium dioxide (TiO2), zirconium dioxide (ZrO2), zinc oxide (ZnO), calcium titanate (CaTiO3), tin (stannic) dioxide (SnO2), molybdenum trioxide (MoO3), and the like. Of this group, titanium dioxide (TiO2) is among the most widely-used of the semiconductor photocatalysts because of its chemical stability, relatively low cost, and an electronic band gap that is suitable for photoactivation by UV light.
The drawbacks to TiO2 photocatalysts currently used as air purifiers is the build-up of products of incomplete oxidation, reduction in performance in humid conditions, mass transport issues in high flow-rate systems, inactivation of the TiO2 photocatalyst, and inorganic contamination.
Titanium dioxide (TiO2) is the most stable oxide form of the transition metal titanium. TiO2 is mostly ionic material composed of Ti+4 cations and O−2 anions. In powder form, TiO2 is white and is widely-used in industry to give whiteness to paint, paper, textiles, inks, plastics, toothpaste, and cosmetics. In crystalline form, TiO2 principally exists as one of three different polymorphic forms: rutile, anatase, and brookite. The two more common polymorphic forms of TiO2, rutile and anatase, have a tetragonal crystal structure, while the less-common brookite form of TiO2 has an orthorhombic crystal structure.
The anatase form of TiO2, which is a low temperature form, has been reported to have the greatest photocatalytic activity of the three polymorphic forms of TiO2 when exposed to UV light. This may be due to a wider optical absorption gap and a smaller electron effective mass in the anatase form that leads to higher mobility of the charge carriers. Anatase is converted to rutile at temperatures above about 600° C., where it is accompanied by crystallite growth and a significant loss of surface area.
The rutile and anatase crystalline structures each have six atoms per unit cell. The anatase form is a body-centered structure and its conventional cell contains two unit cells (i.e., 12 atoms). For both the rutile and anatase forms, titanium atoms are arranged in the crystal structure in such a way that neighboring octahedral units share edges and corners with each other. In the anatase structure, four edges of every octahedral unit are shared edges, as compared within the rutile structure, in which two edges of every octahedral unit are shared edges.
When crystalline TiO2 photocatalyst is irradiated by photons of UV light of less than 387 nm (at room temperature), the band gap energy of TiO2 (3.2 eV) is exceeded, energizing an electron in one of its molecular orbitals to be “promoted” from the valence band into the conduction band of the semiconductor, thereby creating an electron “hole” in the valence band. Electron-hole pairs created in this manner are believed to migrate to the surface where they can initiate redox reactions with contaminants that have adsorbed onto the photocatalyst.
The “promoted” electron eventually recombines with an electron “hole” and returns to the valence band. During the time of electron hole separation, the electron is believed to react with molecular oxygen, and the electron “hole” in the valence band is believed to react with surface hydroxyl groups, forming hydroxyl (.OH) and superoxide radicals, respectively, according to the postulated reactions below:OH−+h+(“hole”)→.OH(hydroxyl radical)O2+e−(“promoted” electron)→.O2−2(superoxide radical)
One of the most active of currently-available TiO2 photocatalysts, such as DEGUSSA P25 (hereinafter “P25”) (Degussa Corporation, Ridgefield Park, N.J., USA) consists of about 80 weight-% of 20 nanometer (nm) anatase TiO2 crystals and about 20 weight-% of larger (about 40 nm) rutile TiO2 crystals. Prepared using a high temperature process, the P25 crystals have a sufficient degree of crystalline perfection to allow sufficient electron hole separation and electron migration to the crystallite surface. The hole at the surface takes the form of a hydroxyl radical (.OH) that is a stronger oxidizing agent than ozone or chlorine. The electron on the surface can form active oxygen species through the reduction of dioxygen, possibly through the formation of superoxide ion, O2− and then by its further reduction to peroxide, O2−2. Hydrogen peroxide is formed over photocatalytically active TiO2 in the presence of oxygen and water. Hydrogen peroxide is believed to be the principal agent of remote photocatalytic oxidation (PCO), which describes the oxidation of substances that are very close to, but not in direct physical contact with, photoactive TiO2. The presence of both hydroxyl radicals and an active oxygen species are needed for effective oxidation of formaldehyde to CO2 and H2O using the anatase form of TiO2.
P25 crystallites having an average crystallite size of about 20 nm and a BET surface area of about 50 m2/gram would seem to be at a theoretical disadvantage as compared with smaller crystallites of TiO2 having an average crystallite size of 10 nm and surface area of greater than 100 m2/g. As used herein, BET (named for the first letters in the surnames of Stephen Brunauer, P. H. Emmett, and Edward Teller, Journal of the American Chemical Society, 1938, vol. 60, pp. 309-319) is a widely-used method in surface science to calculate surface areas of solids by physical adsorption of gas molecules.
Table 1 provides a comparison of average crystallite size with various measures of surface area, including the anatase and rutile forms of TiO2.
TABLE 1Surfacearea/AverageskeletalAvailableSpecificSpecificcrystallitevolume,surface areasurface area,surface areasize, nmm2/cm3m2/cm3m2/g anatasem2/g rutile51200800208188610006671741567857571149134875050013011796674441161041060040010494115453649585125003338778134623088072144292867467154002676963163752506559173532356155183332225852193162115549203002005247212861905045222731824743232611744541242501674339252401604238272221483935292071383632311941293430331821213228351711143027371621082825391541032724401501002623
However, even as Table 1 shows that decreasing average crystallite size of the anatase and rutile forms of TiO2 increases the specific surface area of TiO2, this does not usually result in higher photocatalytic activity, or longer operational life of the photocatalyst, as one would reasonably expect, because heterogeneous catalysis is typically a surface phenomenon. One hypothesis to explain this phenomenon is that the smaller crystallites of TiO2 have imperfections or defects that facilitate rapid electron-hole recombination. See, e.g., Zhang, Z., et al., J. Phys. Chem. B, 1998, vol. 102, pp. 10871-10878, showing that doping anatase TiO2 crystallites with ions such as Fe+3 or Nb+5 increases the photocatalytic activity. These dopants, located within the crystallites, trap the photon-generated electron, thereby retarding electron hole recombination. This hypothesis is supported by the reported relationship between the amount and nature of the dopant (to improve the activity) and anatase crystallite size.
Another factor in the photocatalytic oxidation activity of TiO2 for destruction of VOCs is increased mass transfer resistance that occurs where there is TiO2 having large surface areas but small pore sizes. The small pores limit the photocatalytic activity sites by inhibiting diffusion of VOCs to the active sites.
In cases where the destruction of VOCs results in the formation of a non-volatile ash, such as the oxidation of a siloxane to CO2, H2O and SiO2, the ash can block the active site where the VOC was oxidized, and restricts access to other active sites deeper within the catalytic layer.
Thus, currently-available TiO2 photocatalysts have the disadvantage of poor photocatalytic activity by unit weight, lower activity for eliminating contaminants per unit of readily-available surface area, and short life in actual use.
The present disclosure overcomes these drawbacks of previous TiO2 or doped TiO2 photocatalysts.